Method of chloridizing sintering of zinciferous materials with elimination of lead and similar contaminants



United States Patent METHOD OF CHLORIDIZING SINTERING ZINCIFEROUSMATERIALS WITH ELIMINA- TIAIDNN SOF LEAD AND SIMILAR CONTAMI- N TAugustin L. J. Queneau, Morristown, N. J.

No Drawing. Application June 14, 1952, Serial No. 293,683

Claims. (Cl. 75-5) generally followed at the present time in thepyrometallurgical treatment of zinc sulfide ores, the ore, usually inthe form of a flotation concentrate, is first subjected to adesulfurizing or roasting treatment in which the sulfur content isusually brought down to 3% or less. The roasted ore is then mixed with asmall amount, say 4 to 12%, of a cabonaceous fuel high in fixed carbon,such as coke, anthracite or the like, and with sufficient sodiumchloride or another appropriate chloridizing agent to satisfy thechlorine requirements of the lead, cadmium and any other metal ormetalloid impurities that may be present and which may be causedto'react with chlorine to form chlorides when the ore charge isappropriately heated and subjected to a conventional blast sinteringtreatment.

When the proportion of impurities is relatively small and an appropriatetemperature is maintained during the blast sintering step, as disclosedfor example in my earlier U. S. Pat. No. 1,999,209, it is possible in asingle pass through the sintering machine to eifect the complete removalof such contaminating metals as lead, cadmium, gold, silver andgermanium from an oxidized, i. roasted, zinc ore or concentrate.

It often occurs, however, that zinc ores or concentrates carry higherproportions of lead and cadmium, in some instances up to 5% or more oflead and as much as 2% of cadmium. When zinciferous materials containingupward of about 1% of lead are treated, it becomes increasinglydiflicult to free such materials from lead and cadmium by the methods ofpyrometallurgy without resort to expensive retreating operations. When aclose examination is made to determine what takes place in the course ofa typical chloridizing sintering treatment of zinciferous materialscontaining relatively high proportions of lead and cadmium, it isfound'that the relatively large amounts of lead and cadmium chlorides aswell as other chemical compounds formed during the sintering treatmenttend to condense within the lower part of the charge and especially inthe zone adjacent the iron grate bars of the sintering machine. Thisresults in the rapid clogging of the openings of the grate bars of themachine,

and is followed by the practical cutting off of the air that should bedrawn through the ore bed and the grate bars by the suction fan whichaspirates the air required for the burning of the fuel content of thecharge. The chloridizing sintering operation is thus brought to an endwith the major portion of the metallic impurities, lead, cadice mium,gold, silver, germanium, etc. left in the charge.

Under the usual conditions that obtain in the chloridizing sinteringprocess as heretofore practiced, the successful operation is furtherhindered by the Water required for the thorough mixing and distributionthroughout the charge of the chloridizing and pelletizing agents. Thepresence of water in the pellets at the beginning of the sinteringoperation was also considered to be necessary in order to insure thatthe pellets would become sufficiently permeable to the blast gases whenthe water had been driven 0E in the initial stages of the sinteringtreatment. The heating and vaporizing of this added water as well as anycombined water that may be present absorbs a relatively large percentageofthe available calories liberated by the burning of the carbon and anyother fuel components of the ore charge. This results in an importantlowering of the potential temperature that otherwise would be reached byreason of the combustion of the fuel components of the charge. Underthese conditions the temperature in the wind box of the sinteringmachine may be and usually is near the boiling point of water.

Furthermore, such operating conditions produce an incomplete sinteringof the zinc ore charge, with the result that a large percentage of thecharge is not converted into a satisfactory sinter and falls out as dustat the discharge end of the machine. It has also been proposed tosubject zinc ores carrying varying proportions of lead and other metaland metalloid impurities to a chloridizing sintering treatment whereinthe ore carrying the impurities is disposed on a sintering bed insuperposed relation to a layer of relatively low grade ore and thensubjected to a down draft blast sintering treatment in which the leadand other impurities are vaporized and driven oif from the high gradeore layer and recondensed in the layer of low grade ore. The layer ofhigh grade ore is then scraped off and reduced in the customary manner.However, this process involves handling a relatively large amount ofmaterial and does not bring about a direct recovery of the lead andother metal and metalloid impurities separately from the ore, since theyare in large part concentrated as their chlorides in the layer of lowgrade ore along with the zinc and any other values present in the lowgrade ore. Furthermore, the etfectiveness of the blast diminishes as theporosity of the ore bed decreases due to the condensation of thechlorides.

The present invention has for a principal object to provide an improvedchloridizing sintering process that is particularly adapted for thetreatment of zinc ores that carry relatively high proportions of leadwith or withoutv one or more of the various other metals and metalloidsthat may happen to be present as impurities.

It is a'further object of the invention to provide an improvedchloridizing sintering process wherein it is made possible during thecourse of a hot blast chloridizing sintering treatment of zinc ores orconcentrates carrying a relatively high proportion of lead and similarimpurities to drive ofi and recover relatively free from zincsubstantially all of the lead and similar impurities present in the oreor concentrate supplied to the process.

It is a still further object of the invention to provide an improvedblast roasting and chloridizing sintering operation wherein a raw orunroasted sulfide ore or concentrate is treated in a combined roastingand sintering operation to drive off the sulfur as well as such metalimpurities as lead and cadmium and produce a zinc oxide sinter that issubstantially free of lead and cadmium and of sulfur compounds.

It is still another object of the invention to provide a process ofdesulfurizing raw or unroasted zinc sulfide ores and concentrates on aconventional sintering machine in which the desulfurizing reactions aregreatly speeded up as compared with the practices heretofore followedwithout bringing about undesired fusion or partial melting of the orecharge and without creating undesirable conditions as regardsoverheating of the grates or other metal parts of the; sinteringmachine.

Other objects and advantages ,of the invention will appear as thedescription proceeds.

I have found as a result of extensive investigations that it is possibleto prevent the condensation of the lead and cadmium chlorides in theportions of the ore bed adjacent the suction side thereof withconsequent plugging of the interstices of the charge and the openings ofthe grate bars, provided the temperature within the ore bed, andespecially in the portions thereof adjacent the wind box or suction sideis increased sufficiently to insure that the volatile chlorides, such aslead chloride, that are formed and become vaporized during the sinteringoperation shall remain in vapor form and pass off as such withoutcondensation within the ore bed. My investigations show thatthiscondition will be realized in a down draft chloridizing sinteringoperation when using an appropriately prepared charge by so controllingthe heating of the charge both by the amount of heat units supplied bythe burners and the amount of heat units supplied by the fuel content ofthe charge as to insure that the temperature within the charge on thesintering machine as measured inthe lower part of the ore bed adjacentbut in spaced relation to the grates is brought to at least 1200 C. andpreferably to or even above 1300 C. Stated differently, I have foundthat if the temperature of the gases and vapors leaving the wind box isincreased to 400 C. or over, this will insure that the lead and cadmiumchlorides will remain in vapor form and pass off leaving the remainingzinc oxide sinter substantially completely free of contaminating metaland metalloid impurities.

Under the conditions that obtain in a chloridizing sintering operationcarried on in the conventional Dwight- Lloyd or Greenewalt type ofsintering machine, it is not, however, a simple matter to increase thetemperature of the ore charge to insure an adequate temperaturethroughout the sintering bed. Merely increasing the fuel content of thecharge does not solve the problem. For example, if thecarbon content isincreased to or more, there is a tendency to develop temperatures in thecharge of such magnitude as to bring about softening or even melting ofiron grate bars. Even when more heat resistant metals are substitutedfor the conventional iron grate bars, an inacceptable increase inmaintenance cost may result.

I have found that it is possible to maintainrelatively high temperatureconditions in the ore charge without overheating the grate bars andother metal parts of the sintering machine and at the same time bringabout striking improvements in the operating results, far beyond whatwere foreseeable on the basis of the expected influence of the highertemperature conditions alone, by introducing the modifications in thechloridizing sintering practice that are set out more fully below.

More specifically, I have found that improved results are obtained bylimiting the amount of fuel incorporated in a roasted ore chargeto 4 /2to 6% or thereabouts of carbon or an equivalent amount of sulfidesulfur, based on the dry weight of the ore charge, forming the chargeinto a relatively thin bed of controlled porosity, and then increasingthe amount of heat supplied by the ignition burners or an equivalentoutside source of heat as distinguished from the combustion that takesplace within the charge. When the ore charge is formed .into a bed ofsuitable porosity as more particularly hereinafter described, theoptimum thickness of the bed is a function of the draft produced by thefan and is alsoaffected by the :mechanical condition of the sinteringmachine, such as tightness of the enclosure for the traveling grate andthe presenceorabsence of cracks andopenings through which air may bedrawn into the charge around the burners or directly into the wind boxwithout passing through the ore charge on the grate. These factors alsohave an important bearing on the extent to which heat may be appliedeffectively to the ore charge from the burners, and the more efiicientlythe suction of the fan is applied to draw the products of combustionfrom the burners through the charge the thicker the bed may be and yetinsure a satisfactory continuous progression of the sintering operationwithout plugging of the interstices of the bed with condensed chloridesand other compounds formed during the sintering.

In the operations hereinafter described and which were carried on with atwelve inch wide Dwight-Lloyd sintering machine which allowedconsiderable bypassing of inspirated air around the sintering grate andconsiderably lowered the vacuum that otherwise would have beenobtainable, the most satisfactory sintering runs were conducted with theore charge formed into a bed three inches in thickness. In a full sizedsintering machine in a good condition of repair, the thickness of thebed may be from four to six inches when operating under similar suctionconditions, and by increasing the suction the thickness of the bed maybe increased. However, as contrasted with the present practices in theart of operating with ore charges formed into beds of 12 to 18 inches inthickness, the practice of my invention will usually involve operatingwith relatively thin beds. Nevertheless, the operating advantages of myinvention greatly outweigh the decrease in the capacity of the sinteringmachine considered on a once through basis.

It is a further and important aspect of my invention that the operatingconditions are so maintained as to insure that the products ofcombustion supplied by the ignition burners are caused to be drawnthrough the ore bed by appropriately controlling the suction in relationto the porosity of the charge so that not merely is there an impingement of the flame upon the exposed surface of the charge as itpasses beneath the burners but also the hot combustion gases and anappropriate excess of air are drawn down into and through the bed. Theproportion of fuel supplied by the burners is also very substantiallyincreased as compared with prior practice and the burners are sooperated as to create a powerful stream of combustion gases whichtogether with the excess air bathe and permeate the pellets and rapidlybring the main body of the charge to a relatively uniform temperature.In other words, the burners act not only to rapidly ignite the chargebut also to bring about a much greater and more direct application ofheat to the inner portions of the charge without overheating, since theporosity of the charge insures that the combustion gases and thevolatile compounds formed in the sintering operation are continuouslywithdrawn. The more uniform and higher temperatures created within thebody of the charge also serve to promote vaporization of the chloridesof lead and other contaminants as they are formed and to maintain suchchlorides in the vapor state. In addition, the combustion gases act toentrain the chlorides and other volatile compounds and carry them alonginto the wind box from which they are led to appropriate recovery andseparating apparatus such as electrostatic separators, preferably of thewet type, wherein the chlorides are precipitated and collected.

I have found, for example, that the increased heat input from a sourceoutside of the ore charge may be advantageously brought about byincreasing the burner capacity of the ignition furnace that isordinarily disposed over the headend of the sintering machine and alsousing a fuel gas of high thermal value. Preferably a pair of ignitionburners are disposed adjacent one another at the head or feed-end of thesintering machine. With such a disposition and by supplying appropriateamounts of a high quality fuel such as propane to the burners, it ispossible to develop a'temperature within the ore bed, close to but ins'afely' spaced relation to the grate, of the order of 1200 C. and over,or correspondingly a temperature of 400 C. and over in the wind box,provided the ore is suitably pelletized and the voids and intersticesbetween the ore pellets or agglomerants are not allowed to plug up withcondensed chlorides.

In order further to promote relatively high temperatures throughout thecharge and particularly in the portion thereof close to the grate bars,I remove substantially all moisture from the charge after the ore hasbeen pelletized and preferably while the ore chargeis disposed in situon the bed of the sintering machine. Advantageously, the sinteringmachine may be modified by introducing a drying section ahead of thesintering hearth with a separate suction fan connected to the dryinghearth and discharging directly to the atmosphere. The drying hearthwill usually be provided with burners to supply heat to promote thedrying action, but ignition of the combustible components of the chargeis not initiated in this stage. Instead of providing a drying section inthe sintering machine ahead of the ignition burners, the pellets may bedried in a known type of dryer before they are fed to the sinteringmachine.

In preparing zinc ores or concentrates for the chloridizing sinteringtreatment, I also find that it is advantageous as a preliminary to thepelletizing step, to add to the otherwise suitably proportioned mixtureof zinc ore, carbon and chloridizing agent (preferably sodium chloride)from 2 /2 to 6% by weight of crude zinc sulfate or ferrous sulfate,based on the dry weight of the charge, together with a suitableproportion of sinterfines or of crushed sinter as returns. An amount ofchloridizing agent approximating 100 to 150% of the stoichiometricrequirement for converting all of the lead and cadmium into chlorideswill be ordinarily used, but this proportion may be'increased in specialcases. The chloridizing agent and the sulfate addition are preferablydissolved in water before addition to the other ingredients of thecharge.

The proportion of returned sinter fines or crushed sinter may be variedconsiderably and may be omitted entirely in the initial chloridizingsintering step when the process is being practiced by subjecting the oreto more than one pass through the sintering machine. Usually whentreating an ore containing around 2-3 of lead, an addition of sinterfines and/or crushed sinter equal to 25% to 50% of the dry weight of thecharge is recommended. The proportion should be increased when treatingores containing higher proportions of lead and cadmium since thisreduces the tendency of the chlorides of these metals to recondense inthe ore bed rather than to be carried away in the blast gases drawnthrough the charge.

Following the various additions the mixture is pelletized in a drumpelletizer. The pellets are then screened and those larger thanone-fourth inch are crushed and returned to the pelletizing drum. Theore charge is then dried to the point where substantially all moistureis eliminated. Preferably the drying is carried on after the pelletshave been fed to the sintering machine and while they are disposed insitu on the sintering bed.

EXAMPLE 1 Roasted are Roasted zinc concentrate, 16 mesh 58 lbs. Crushedsinter returns 42 lbs. Anthracite coal (89% fixed carbon, 2%

sulfur) lbs. Sodium chloride 4 lbs. 5 ozs.

Ferrous sulfat Water 6 lbs. 6 ozs. 7 lbs.,

a The charge was mixed in a pelletizing drum, and the" resulting pelletswere then dried and screened over a 4-' mesh sieve. The pellets werethereafter fed to a laboratory twelve-inch Dwight-Lloyd sinteringmachine and deposited on a previously prepared hearth layer of one-halfinch thickness composed of plus one-fourth inch sinter from a previousrun on the same type of concentrate. This sinter had been produced undersomewhat different operating conditions and contained 0.29% lead and nocadmium. The thickness of the ore charge was three inchesinclusive ofthe one-half inch hearth layer.

The machine as operated gave a suction through the bed equivalent to avacuum of fourteen and one-half inches of water, and the traveling gratemoved at the rate of one and one-half inches per minute first past twohigh intensity ignition burners supplied with propane gas and positionedone after the other at the feed end of the machine with their centersdisposed 20 inches apant and then along the rest of the length of themachine. A higher vacuum would have been desirable but was notattainable with the condition of the machine as it was. The first palletwas taken off the machine 34 minutes after the beginning of the run andthe last pallet was removed 1 hour and 6 minutes after the start of theopera-tion. The sinter produced had a very good appearance and came offthe pallets in good cakes. A sinter was formed all the way to thegrates, and the grate bars stayed very clean. The sintering wasaccompanied by very heavy fuming. A temperature of 382 C. was attainedin the wind box and the highest stack temperature recorded was 227 C.The temperature of the charge as measured 1" above the level of thegrate was 1299 C. Analysis of the sinter showed that only 0.01% leadremained at the conclusion of the run. The cadmium had been completelyremoved. A small amount of fines, 5 /2 lbs., from the sinteringoperation assayed 0.22% lead. The yield of good sinter was lbs.

EXAMPLE 2 Roasted are 2% sulfur, crushed to 10 mesh 15 Ferrous sulfate18 Sodium chloride 9 Water 30 The ferrous sulfate and sodium chloridewere dissolved in water and then mixed with the other ingredients in apelletizing drum. The pellets were then dried sufi'iciently to permit ofscreening and then were screened over a 4-mesh sieve. The pelletspassing the 4-mesh sieve were then fed to the sintering machine anddisposed on the sintering bed as a layer two and one-half inches thickin superposed relation to a one-half inch layer of relatively coarsechloridized sinter from a previous operation. This bottom layerconsisted of sinter granules which remained on a 4-mesh sieve. were thenfurther dried in situ by causing hot air to be drawn through the chargeuntil substantially all moisture was removed. Thereafter the charge wassintered under similar operating conditions as were maintained in thetest previously described. The resulting sinter :car-r-ied only 1.0%lead and no cadmium.

A second run was then made with an ore charge pro- 75, pontioned'asfollows:

The moist pellets,

Pounds Roasted zinc concentrate of the same composition and fineness asused in the first test under this example above described 15'0 Sinterreturns, i. e., the sinter produced in the first test of this series andcrushed to 18 mesh 150 Anthracite coal 18 Ferrous Sulfate 15 Sodiumchloride 7 Water 30 After mixing and pelletizing in the same manner asdescribed above, the pellets were screened and the -4 mesh pellets werefed to the sintering machine and thoroughly dried in situ tosubstantially completely remove .all water present therein. They werethereafter subjected to sintering under similar conditions as weremaintained in the next previous test. The sinter resulting from this runcontained 0.67% lead with no cadmium.

In order to further purify the concentrate with respect to lead, thesinter from the second test of this series was crushed and rc-sinteredafter first mixing and pelletizing with fuel and reagents in thefollowing proportions:

Sinter from second test, crushed to l8 mesh 270 lbs. Anthracite coal 12lbs. Ferrous sulfate 13 /2 lbs. Sodium chloride 2 lbs. 6 ozs. Water 19lbs.

As before, the pellets were reduced to complete dryness after being fedto the sintering machine and before initiating the blast-sinteringoperation. The charge was then sintered under the same operatingconditions as were maintained in the first two tests of this series.sulting sinter contained only 0.04% lead. It was also noted that thesulfur, both sulfide and sulfate sulfur, 'had been completely removed.

In all of the tests carried on under this example and described above,double ignition burners supplied with propane gas were used and the heatinput was regulated to produce a temperature of about 1300 C. asmeasured by thermocouples inserted in the charge about one-half inchabove the sintering grate.

Combined desulfurizing and chloridizing sintering The invention is alsoapplicable in the treatment of raw, unroasted zinc sulfide ores andconcentrates, and has the advantage over the prior practice ofdesulfurizing ofraw zinc sulfide ore and concentrates on a sinteringmachine, sometimes referred to as the Robson process, that it ispossible to speed up very materially the desulfurizing operation. Stateddifferently, it is possible materially to reduce the number of sinteringtreatments required to bring about elimination of the sulfur.

According to the prior practice of desulfurizing raw zinc sulfideconcentrates on a sintering machine, it is customary to conduct theoperation so as to remove from 4 /2 to sulfur units in a single passagethrough the sintering machine; otherwise, there is danger that theoxidation reaction will generate. so much heat. as to fuse or partlymelt the ore charge and thus hinder further reduction of the sulfurcontent of the. ore. Consequently, under the Robson practice it isusually necessary to pass the ore five or more times through thesintering machine when it is the usual type of zinc sulfide concentratecarrying 30 to 31% sulfur. In the Robson process control of theoxidation is usually brought about by mixing the raw zinc sufideconcentrate with sufiicient previously desulfurized sinter of the sameor a similar ore to reduce the sulfur present to a suflicient amount forthe blast roasting operation. It will be appreciated that this involvestreating over and over again a relatively large amount of materials andrequires a relatively large sintering machine capacity for a givenoutput of roasted concentrate. i

The re- The following is a typical example of an operation of my processas applied to the treatment of raw zinc sulfide ore concentrates:

EXAMPLE 3 Raw (unroasted) zinc sulfide concentrate- 58 lbs. Sinterreturns (roasted and chloridized ore) 42 lbs. Ferrous sulfate 3 lbs. 2ozs. Salt 4 lbs. 5 ozs. Water .7 lbs.

As in the tests described earlier in this specification, the ferroussulfate and salt was dissolved in the water before addition to the orecharge. After mixing, pelletizing and screening, the screened pelletswere fed to a sintering machine provided with a drying hearth sectionwhere they were first subjected to a controlled drying treatmentdesigned to drive off the water and reduce the charge to substantiallycomplete dryness while the pellets were disposed in situ 0n thetraveling bed. The dry charge was then passed to the head of thesintering section proper and there subjected to a blast-roastingtreatment under similar conditions as to heat input and other operatingfactors as were maintained in the operations using a roastedconcentrate. Likewise as in the tests previously described, the overallthickness of the ore bed was kept at about three inches with a bottomlayer of previously roasted and chloridized sinter of about one-halfinch thickness and consisting of sized particles insuring adequateporosity, plus 4-mesh. The product resulting from the sintering run overthe sintering machine carried only from 0.01 to 0.02 lead and no cadmiumand was substantially free from sulfur. Analysis of the fume recoveredin this operation showed a lead content of 33.56% with 22.0% cadmium.The proportion of zinc was not precisely determined but was estimated tobe in the neighborhood of 12%.

A sinter from a previous chloridizing sintering run using the same oreor concentrate is preferably used in forming the bottom layer. This hasthe advantage that it is not necessary to effect any separation of thesinter as between the portion disposed in the bottom layer and thatproduced in the superimposed portions of the charge. However, it will beunderstood that the desulfurizing and chloridizing reactions willproceed to the same extent regardless of the nature of the bottom layerprovided it is sufficiently coarse and porous to be readily permeable bythe blast gases and serves to support the sulfides of the charge inspaced relation to the sintering grates.

It was observed that in all cases, whether the zinciferous material hadbeen previously roasted or was in the form of raw sulfides, theproportion of zinc carried over in the fume was relatively low ascompared with the amount carried over in the fume from a conventionalchloridizing sintering operation conducted according to the prior artcommercial practices.

A further advantage resulting from the practice of my invention in thecombined desulfurizing and chloridizing sintering of zinc sulfide oresand concentrates resides in the fact that, in contradistinction to theRobson process and similar practices, the sulfur-containing gasesproduced in the combined desulfurizing and chloridizing sintering stepmay contain 8 to 10% sulfur without any re-cyoling and are, therefore,sufficiently rich in sulfor to make them suitable for use in themanufacture of sulfuric ,acid either by the chamber or the contactprocess. Even when a somewhat higher proportion of sinfer returns isused than was the case in Example 3, the concentration of sulfur dioxidein the gases passing along with the fume will be suificiently high tomake it commercially feasible to recover the surfur dioxide for use inthe manufacture of sulfuric acid. Usually, however, when it is aimed toutilize the sulfur content of the gases, it is preferred to regulate theproportion of sinter returns in relation to the sulfur content of theunroasted sulfide ore or concentrates so as to insure that the chargeshall contain at least of sulfur as the sole fuel component of thecharge. It will be understood, of course, that all of the chlorides andother objectionable impurities must be removed from the gases beforethey are used to make sulfuric acid.

It will be understood that various changes may be made in the details ofthe procedures outlined above without departing from the invention whichis not to be deemed as limited otherwise than is indicated by thelanguage of the appended claims. For example, reference has been madeherein to the addition of zinc sulfate or ferrous sulfate to aid inbinding the ore particles during the pelletizing step. It is to beunderstood that other binding agents may be employed which do not giverise to undesired reactions in the subsequent desulfurizing andchloridizing sintering treatment or a sub sequent chloridizing sinteringtreatment alone, as the case may be. As examples of other suitablebinding agents mention is made of molasses, flour, waste sulfiteliquors, bentonite and other clays.

It will also be understood that the drying of the pelletized orepreliminary to the sintering step may be omitted, in which case theduration of the blasting operation will usually need to be prolonged orelse additional fuel will need to be provided in the charge. However,the operation is speeded up and very much improved by subyecung the orepellets to drying to substantially complete dryness either in situ or insome other suitable manner.

It is to be understood that the term ore as used in this specificationand in the appended claims is inclusive of the zinc ores in theirnatural state and also in their beneficiated form customarily referredto as concentrates.

I claim:

1. In the process of forming a chloridized sinter of zinciferousmaterial containing in excess of 1% of lead to eliminate lead and anyother associated metal and metalloid impurities therefrom wherein thezinciferous material is prepared for the sintering treatment by,subjecting a mixture of said ore, fuel and a chloridizing agent to apelletizing treatment with addition of sufficient water and a binder tobind the mixture into self-sustaining pellets; the improvement whichconsists in increasing the porosity of the charge in a step preliminaryto the ignition of the charge by drying the pellets to substantialdryness while disposed in situ on a sintering grate and therebyconcomitantly decreasing the fuel requirements of the charge, thereafterapplying a blast flame to the upper side of said charge and applyingsuction at the grate side to draw air and combustion products throughsaid charge while the charge is disposed in a relatively thin layer onthe sintering grate, the suction and the amount of fuel being suppliedto the blast flame being regulated so as to maintain a temperature of atleast 1200 C. as measured approximately one inch from the bottom of thebed as it is disposed on the sintering grate but below the temperatureat which fusion of the charge will occur, thereby chloridizing andvaporizing the lead and other associated metal and metalloid impurities,and withdrawing said vaporized chlorides as formed.

2. In the recovery of zinc from zinciferous materials containing, inaddition to zinc at least 1% of lead and other associated metalimpurities, the improvement which comprises admixing the zinciferousmaterial with a chloridizing agent, from 4 to 6% carbon, and sufficientwater and a .binder to form the mixture into self-sustaining pellets,forming the charge into pellets, disposing said pellets in a layerhaving a thickness of 4 to 6 inches on a sintering bed in superposedrelation to a highly porous layer of sinter of sufficient porosity toinhibit plugging of the grate apertures and sufficient thickness toprevent 'overheating of the sintering grate during the blasting step, ina step preliminary to the ignition of the charge drying the pellets tosubstantially complete dryness while disposed in situ on said bed,directing an oxidizing flame onto the exposed surface of said bed whileapplying suction to the opposite side thereof, and regulating theoxidizing flame in relation to the thickness and porosity of the bed andto the reduced pressure created at the opposite side of said bed so asto insure a relatively uniform temperature throughout the body of saidcharge and a temperature in the lower part of said charge taken atapproximately one inch from the grate of about 1200-1300 C., saidoxidizing flame being further so regulated as to prevent fusion of thecharge, thereby driving off lead and other associated metal chloridessubstantially as fast as formed, and recovering said chlorides.

3. In the recovery of zinc from its sulfide ores the method ofdesulfurizing and chloridizing said ore in a single operation withsimultaneous elimination of lead and any other associatedmetalandmetalloid impurities, which comprises mixing said ore, previouslyreduced to finely divided form with finely divided substantiallydesulfurized zinc oxide sinter and with sufficient chloridizing agent tosatisfy the chlorine requirements of the non-zinc metal impurities in asubsequent chloridizing blast-sintering treatment of the charge,regulating the proportion of sulfide ore in said charge so as to provideat least 15% of sulfur as the sole fuel component of the charge,subjecting the mixture to a pelletizing treatment with addition ofsufiicient water and a binder to bind the mixture into self-sustainingpellets, disposing said pellets in a bed of 4 to 6" thickness on asintering grate, in a step preliminary to ignition of the charge passingheated gases through said bed until the pellets have been reduced tosubstantially complete dryness, and thereafter simultaneouslychloridizing and blastsintering the pelletized ore by directing anoxidizing flame on to the exposed surface of said bed while applyingsuction to the opposite side thereof to maintain a temperature of atleast 1200 C. as measured approximately one inch from the bottom of thebed as it is disposed on the sintering grate but below the temperatureat which fusion of the charge will occur.

4. In the recovery 'of zinc from zinciferous materials containing inexcess of 1% of lead, the method of eliminating lead and otherassociated metal and metalloid impurities therefrom which comprisesoxidizing the metal values of the ore, crushing the oxidized ore andadding a chloridizing agent and suflicient carbon to bring the fuelcontent of the charge within the carbon equivalent range of 4 to 6%based on the dry weight of the ore charge, subjecting the mixture to apelletizing treatment with addition of sufficient water and a binder tobind the mixture into self-sustaining pellets, in a step preliminary toignition of the charge heating said pellets under drying conditions todrive off substantially all of the free water contained therein,disposing the pellets as a bed on a traveling sintering grate,thereafter simultaneously chloridizing and blastsintering said bed ofpelletized ore by applying a blast flame to the upper side of said bedand applying suction at the grate side to draw air and combustionproducts through said bed, regulating the suction and the amount of fuelsupplied to the blast flame to establish and maintain a temperature ofat least 1200 C. as measured approximately one inch from the bottom ofthe bed as it is disposed on the sintering grate but below thetemperature at which fusion of the charge will occur, therebychloridizing and vaporizing substantially all of the lead and otherassociated metal and metalloid impurities, and withdrawing saidvaporized chlorides as formed.

5. The method according to claim 4 wherein the thickness of the orecharge on the sintering bed is maintained within the range 4 to 6inches.

6. In the recovery of zinc from its ores containing in excess of 2%lead, the method of eliminating lead and other associated metalandmetalloid impurities therefrom which comprises oxidizing the metalvalues of the ore, crushing the oxidized ore and admixing therewith asubstantial proportion of sinter fines relatively low in lead and othermetal impurities'obtained from a preceding chloridizing sinteringtreatment, adding a chloridizing agent and sufficient carbon to bringthe fuel content of the charge within the carbon equivalent range of 4to 6% based on the dry weight of the ore charge, subjecting the mixtureto a pelletizing treatment with addition of sufii cient water and abinding agent to bind the mixture into self-sustaining pellets, in astep preliminary to ignition of the charge heating the pellets whiledisposed in situ on a traveling sintering grate to drive offsubstantially all of the water, thereafter simultaneously chloridizingand blastsintering the pelletized ore by applying a blast flame to theupper side of said bed and applying suction at the grate side to drawair and combustion products through said bed, regulating the suction andthe amount of fuel supplied to the blast flame to establish and maintaina temperature of at least 1200 C., as measured approximately one inchfrom the bottom of the bed as it is disposed on the sintering grate, butbelow the temperature at which. fusion of the charge will occur, therebychloridizing and vaporizing substantially all of the lead and otherassociated metal and metalloid impurities, and withdrawing saidvaporized chlorides as formed.

7. In the recovery of zinc from its sulfide ores, the method ofdesulfurizing said ore with simultaneous elimi nation of lead and anyother associated metal and metalloid impurities which comprises mixingsaid ore previously reduced to finely divided form with finely dividedsubstantially desulfurized zinc oxide sinter and with sufficientchloridizing agent to satisfy the chlorine require ments of the lead andany other associated metal and metalloid impurities in a subsequentchloridizing blast-sinterin g treatment of the ore, the proportion ofdesulfurized sinter being sufficient to prevent overheating of thecharge during the subsequent blast-sintering treatment, subjecting themixture to a pelletizing treatment with addition of sufficient water anda binder to bind the mixture into selfsustaining pellets, removing anyfines and oversized pellets, disposing the remaining pellets on atraveling sintering grate, in a step preliminary, to ignition of thecharge heating the pellets under drying conditions in situ as a bed onsaid grate to drive off substantially all of the moisture presenttherein, thereafter simultaneously chloridizing and blast sintering thepelletized ore by applying a blast flame to the upper side of said bedand applying suction at the grate side to draw air and combustionproducts through said bed, regulating the suction and the amount of fuelsupplied to the blast flame to establish and maintain a temperature ofat least 1200 C., as measured approximately one inch from the bottom ofthe bed as it is disposed on the sintering grate, but below thetemperature at which fusion of the charge will occur, therebychloridizing and vaporizing substantially all of the lead and otherassociated metal and metalloid impurities, and withdrawing saidvaporized chlorides as formed.

8. In the recovery of zinc from its ores containing in excess of l% oflead, the method of eliminating lead and any other associated metals andmetalloid impurities therefrom which comprises admixing the ore with achloridizing agent, carbon and sufficient water and a binder to form themixture into selfsustaining pellets, forming the mixture into pelletsand disposing said pellets in a relatively thin layer on a sintering bedin superposed relation to a previously deposited highly porous layer ofsintered ore of sufiicient thickness and porosity to inhibit plugging ofthe grate apertures, and prevent overheating of the sintering gratesduring the blasting step, in a step preliminary to ignition of thecharge drying said pellets to sub stantially complete dryness whiledisposed in situ on said bed, and thereafter sintering said pellets byapplying an oxidizing flame to the upper side of said bed and applyingsuction at the grate side to draw air and combustion products throughsaid bed, and regulating the suction and the amount of fuel supplied tothe blast flame to establish and maintain a temperature of at least l200C., as measured approximately one inch from the bottom of the bed as itis disposed on the sintering grate, but below the tem perature at whichfusion of the charge will occur, thereby chloridizing and vaporizingsubstantially all of the lead and other associated metal and metalloidimpurities, and withdrawing said vaporized chlorides as formed.

9. In the recovery of zinc from zinciferous materials containing, inaddition to zinc, in excess of 2% of lead and also cadmium and othermetal impurities, the improvement which comprises admixing with thezinciferous material a zinc oxide sinter relatively low in lead andother metal impurities, adding a chloridizing agent in an amountsufiicient to satisfy the chlorine requirements of the non -zinc metalimpurities in a subsequent chloridizing sintering treatment of thecharge, and so regulating the proportions of zinc oxide sinter and offuel present in the charge as to insure that the temperatures developedwithin the charge during a blast sintering operation will not causefusion of the charge, subjecting the mixture to a pelletizing treatmentwith addition of sufficient water and a binder to bind the mixture intoself-sustaining pellets, in a step preliminary to ignition of the chargedrying the pellets to substantial dryness While disposed in situ as abed in a relatively thin layer on a traveling sintering grate,thereafter simultaneously chloridizing and blast-sintering thepelletized ore by applying a blast flame to the upper side of said bedand applying suction at the grate side to draw air and combustionproducts through said bed, regulating the suction and the amount of fuelsupplied to the blast flame to establish and maintain a temperature ofat least 1200 C., as measured approximately one inch from the bottom ofthe bed as it is disposed on the sintering grate, thereby chloridizingand vaporizing the lead and other associated metal and metalloidimpurities, and Withdrawing said vaporized chlorides as formed.

10. In the recovery of zinc from its sulfide ores, the method ofdesulfurizing and chloridizing said ore in a single operation withsimultaneous elimination of lead and any other associated metal andmetalloid impurities, which comprises mixing said ore previously reducedto finely divided form with finely divided substantially desulfurizedzinc oxide sinter and with sufficient chloridizing agent to satisfy thechlorine requirements of the lead and any other associated metal andmetalloid impurities in a subsequent chloridizing blast-sinteringtreatment of the ore, regulating the proportion of sulfide ore in saidcharge so as to provide at least 15% sulfur as the sole fuel componentof the charge, subjecting the mixture to a pelletizing treatment withaddition of sufiicient water and a water dispersible binder to bind themixture into self-sustaining pellets, disposing the pellets in arelatively thin bed on a sintering grate, in a step preliminary toignition of the charge drying the pellets to substantial dryness, andthereafter simultaneously chloridizing and blast-sintering thepelletized ore by applying a blast flame to the upper side of said bedand applying suction at the grate side to draw air and combustionproducts through said bed, regulating the suctron and the amount of fuelsupplied to the blast flame to establish and maintain a temperature ofat least 1200 C., as measured approximately one inch from the bottom ofthe bed as it is disposed on the sintering grate, but below thetemperature at which fusion of the charge will occur, therebychloridizing and vaporizing the l d d other 13 associated metal andmetalloid impurities, and Withdrawing said vaporized chlorides asformed.

References Cited in the file of this patent UNITED STATES PATENTS976,525 Wedge NOV. 22, 1910 1,058,034 Christensen Apr. 8, 1913 1,463,901Maxson Aug. 7, 1923 1,773,991 Gerlach Aug. 26, 1930 10 1,862,828 RobsonJune 14, 1932 14 Gonser Sept. 12, 1933 Queneau Apr. 30, 1935 WendebornAug. 25, 1936 Levison Dec. 15, 1936 Hyde May 3, 1938 Robson et a1 Oct.4, 1938 Orr et a1 Sept. 30, 1952 Burrow et a1. Mar. 16, 1954 FOREIGNPATENTS Great Britain Aug. 22, 1935

1. IN TH PROCESS OF FORMING A CHLORIDIZED SINTER OF ZINCIFEROUS MATERIALCONTAINING IN EXCESS OF 1% OF LEAD TO ELIMINATE LEAD AND ANY OTHERASSOCIATED METAL AND METALLOID IMPURITIES THEREFROM WHEREIN THEZINCIFEROUS MATERIAL IS PREPARED FOR THE SINTERING TREATMENT BYSUBJECTING A MIXTURE OF SAID ORE, FUEL AND A CHLORIDIZING AGENT TO APELLETIZING TREATMENT WITH ADDITION OF SUFFICIENT WATER AND A BINDER TOBIND THE MIXTURE INTO SELF-SUBSTAINING PELLETS; THE IMPROVEMENT WHICHCONSISTS IN INCREASING THE POROSITY OF THE CHARGE IN A STEP PRELIMINARYTO THE IGNITION OF THE CHARGE BY DRYING THE PELLETS TO SUBSTANTIALDRYNESS WHILE DISPOSED IN SITU ON A SINTERING GRATE AND THEREBYCONCOMITANTLY DECREASING THE FUEL REQUIREMENTS OF THE CHARGE, THEREAFTERAPPLYING A BLAST FLAME TO THE UPPER SIDE OF SAID CHARGE AND APPLYINGSUCTION AT THE GRATE SIDE TO DRAW AIR AND COMBUSTION PRODUCTS THROUGHSAID CHARGE WHILE THE CHARGE IS DISPOSED IN A RELATIVELY THIN LAYER ONTHE SINTERING GRATE, THE SUCTION AND THE AMOUNT OF FUEL BEING SUPPLIEDTO THE BLAST FLAME BEING REGULATED SO AS TO MAINTAIN A TEMPERATURE OF ATLEAST 1200* C. AS MEASURED APPROXIMATELY ONE INCH FROM THE BOTTOM OF THEBED AS IT IS DISPOSED ON THE SINTERING GRATE BUT BELOW THE TEMPERATUREAT WHICH FUSION OF THE CHARGE WILL OCCUR, THEREBY CHLORIDIZING ANDVAPORIZING THE LEAD AND OTHER ASSOCIATED METAL AND METALLOID IMPURITIES,AND WITHDRAWING SAID VAPORIZED CHLORIDES AS FORMED.